The present invention relates to a ceramic wiring board. Particularly, this invention relates to a ceramic board having a low dielectric constant, which can be favorably employed as a circuit wiring board in an electronic computer, etc.
It is known that the propagation speed of an electric signal in a circuit wiring board, etc. is, in general, closely related to the length l of a metal for wiring conductor and the capacity C of an insulating wiring layer. The capacity C is substantially proportional to the dielectric constant .epsilon. of the insulating layer. Thus, the lower the dielectric constant, the higher the electric signal propagation speed in the wiring board. More specifically, the signal propagation speed is inversely proportional to the square root of the dielectric constant .epsilon..
In general, it is required that the electric signal propagation speed be as high as possible. Thus, a substance having a low dielectric constant is currently desired as the wiring board material. Particularly in the field of large electronic computers where a higher speed is very earnestly demanded, a decrease in the dielectric constant of a ceramic board for mounting parts thereon is eagerly desired.
Organic substances such as resins were employed as wiring board materials of old. Since, however, they are poor in reliability, ceramics such as alumina have recently been employed. However, the dielectric constant of alumina (Al.sub.2 O.sub.3) is about 9, which is unsuitable for the above-mentioned requirement of a low dielectric constant. In view of this, intensive investigations have been made on application of materials having a lower dielectric constant than that of Al.sub.2 O.sub.3 to wiring boards. Examples of such materials include mullite ceramics (dielectric constant: 6 to 7) and glass ceramics (dielectric constant: 5 to 6). Although the dielectric constants of these materials are lower than that of Al.sub.2 O.sub.3, they are yet insufficient as those of high-speed propagation board materials.
The material having the lowest dielectric constant among ceramics is silica (SiO.sub.2) (dielectric constant: about 4). Thus, if silica is used, a wiring board capable of high-speed propagation of signals can be expected. However, in realizing a further improvement in the performance of a board, SiO.sub.2 itself has limitations in achieving a low dielectric constant. Thus another measure must be taken for realizing a lower dielectric constant.